1. Field of the Invention
Certain inventive aspects relate to non-destructive optical measurement techniques for determining the dopant carrier profile in semiconductor layers, using optical energy to create charge carriers in these semiconductor layers and to probe changes in reflectivity created by these charge carriers. More particularly, certain inventive aspects relate to a method for the independent extraction of the peak dopant concentration and junction depth in a particular semiconductor substrate, from a single measurement as well as devices and software for carrying out the methods.
2. Description of the Related Technology
In semiconductor processing, methods are required for the determination of properties of semiconductor materials, such as Si, SiGe, GaAs, . . . , and their dependence on processing conditions. Introducing species into a semiconductor material by, for example, ion implantation can change the properties of the bulk material. Other methods that can change the properties of the bulk material are manufacturing of the substrate, annealing such as for example rapid thermal processing (RTP) or rapid thermal annealing (RTA), etc.
In CMOS (Complementary Metal Oxide Silicon) devices for example, it is important to be able to determine the junction depth and profile of the source and drain regions formed in the semiconductor substrate. For advanced high-performance Complementary Metal-Oxide-Semiconductor (CMOS) technologies, it is, for example, crucial to be able to quickly and reliably characterize ultra shallow junctions. As CMOS structures, such as for example transistors, become increasingly smaller (<50 nm), dopant profiles shrink accordingly (unction depths less than 20 nm) and their exact determination becomes more difficult and at the same time more critical. Process conditions can be optimized to obtain the desired junction depth and profile and, hence, to obtain the required device characteristics.
Various methods exist to investigate the properties of the semiconductor dopant profile. Some of these techniques, however, are destructive. One example of such destructive techniques is spreading-resistance-profile (SRP), whereby a semiconductor substrate to be characterized is cleaved along a diagonal cleavage line and a two point electrical measurement is then performed at subsequent positions along this cleavage line. Other known techniques are non-destructive techniques such as, for example, the Carrier Illumination™ (CI) technique, as disclosed in U.S. Pat. No. 6,049,220 and U.S. Pat. No. 6,323,951, both hereby incorporated by reference in their entirety. For example for in-line monitoring of the pre- and post-anneal process steps, this Carrier Illumination™ technique has established itself as a fast, non-contact, non-destructive tool having wafer mapping capability. For process monitoring applications, the exact quantitative interpretation of the CI-signal is less important as long as high repeatability and sensitivity for a particular profile or process parameter can be demonstrated.
The CI technique uses optical reflectance to measure junction depth. It is based on the linear decrease (β<0) of the index of refraction of a semiconductor with the carrier concentration, as expressed by equation (1):n=n0+βN  (1)wherein:                n is the index of refraction of the doped semiconductor material,        no is the index of refraction of the semiconductor material in absence of a carrier concentration (being for example 3.42 for silicon),        N is the free carrier concentration, and        β is a constant.        
In practice, the concentration-dependent term is in the range of 5.10−3 or less, too low thus to contrast the doped layer against the substrate. The CI technique takes advantage of excess carrier pile-up at the edge of the doped layer to obtain sufficient contrast. By illuminating the doped region, excess carriers are created. In the quasi-static regime in which CI operates, the excess carriers move through diffusion and drift. The concentration of excess carriers, which is determined by the difference in carrier concentration with and without illumination, rises steeply at the edge of the doped region.
In CI and similar techniques such as, for example, the Thermo-Probe technique, typically, two lasers are used. A first laser is a focused pump laser or generation laser, generating a “pump” laser beam or generation beam. The first laser operates at a fixed wavelength, which is larger than the band gap of the semiconductor material under study and typically is about 830 nm. This laser is used to generate a quasi-static excess carrier profile in the bulk of the semiconductor material, giving rise to a depth dependent index of refraction of the material. The excess carriers distribute themselves in the semiconductor material according to a profile which is defined as the carrier concentration and is expressed in number of carriers per cm3 exceeding the level of carriers present within the semiconductor substrate without stimulation, this latter being labelled as the background carrier concentration or profile, e.g. in the absence of illumination. This background carrier concentration is dependent on the concentration of dopant atoms. Specifically, the excess carrier concentration changes from zero outside a surface of the semiconductor material to a finite value inside the semiconductor material. This change in excess carrier concentration results in a steep increase in the concentration of excess carriers at the surface of the semiconductor substrate. This steep increase of the excess carriers concentration at the interface between the semiconductor material under study and its surroundings, e.g. air, will be labelled as the near-surface component which will result in a near-surface component of a reflected probe beam as will be discussed later on. As the depth z, which is defined from the front surface of the semiconductor substrate into the semiconductor substrate, increases, the excess carrier concentration changes proportionally to the change in the concentration of dopant atoms or to the presence of recombination centers. For example, in some cases, the dopant concentration rises, but in other cases the dopant concentration dips first and then rises, depending on the detailed shape of the doping profile.
In the CI technique, a CI-signal to be measured is then generated by illuminating the optically stimulated semiconductor material with a second “probe” laser, generating a probe laser beam or probe beam, which may also be labelled analyzer beam, having a fixed wavelength which is higher than the fixed wavelength of the “pump” laser and typically is about 980 nm. This probe laser beam will be reflected at the sample surface and/or at any region with a large change in the index of refraction proportional to the excess carrier profile, as is illustrated in FIG. 1 (see below). Reflected light from the second laser provides a profile depth signal. Use of photon energy at or below the band gap of the material under study minimizes excess carrier generation by the second laser, so that the power of the first laser exclusively determines the excess carrier profile for sufficiently high power settings.
Reflected signals are converted to a value representative of junction depth using an algorithm developed through extensive correlation of CI-measurements with SRP measurements on a wide range of implants. The primary parameter reported is the profile depth at a pre-selected concentration, for example, 1×1018/cm3. Algorithms are available for n/p, p/n and p/p structures.
FIG. 1 shows a semiconductor substrate 1 and a probe laser beam 3 impinging from the surroundings 2 on the semiconductor substrate 1. The incident probe laser beam 3 and reflected probe laser signal 4 are indicated by respectively arrows 3 and 4. The semiconductor substrate 1 comprises a doped layer 1a formed on an undoped or lower doped region 1b. The substrate 1 can be formed by depositing an in-situ doped layer 1a on top of layer 1b, yielding a uniform doping profile over region 1a or can be formed by implanting dopants into the substrate 1, yielding a doped region 1a and an undoped region 1b. 
By using e.g. ion implantation for implanting dopants into the substrate 1, any kind of doping profile can be obtained depending on the choice of implant species, the energy and implantation dose used. Layer 1a can be doped with a dopant of the same or the opposite type of dopant used to dope the underlying layer 1b. In FIG. 1, the excess carrier profile N(z) as function of depth z into the substrate 1 is also shown, indicated by graph 5. The probe laser beam (arrow 3) will be reflected, thus generating the reflected probe laser signal (arrow 4) at various positions on the semiconductor substrate 1. For example, the probe laser beam 3 may be reflected at the surface, yielding a surface component in the reflected probe laser signal 4. It may also be reflected by a change in the excess carrier profile which can occur at the surface, yielding a near-surface component, or at the interface between the doped part 1a and undoped part 1b on the gradient of N(z), yielding a bulk component. The original purpose of the measurement is to extract from the total reflected signal 4 the reflected probe signal originating from the bulk of the device, as only the remaining signal will give information about the doping profile. Therefore, ideally the surface and the near-surface components should be eliminated from the total reflected signal 4.
Laser beams from both lasers, pump laser and probe laser, are superimposed onto each other and may contact the semiconductor substrate 1 in the same or in a different area. Typically, both lasers are in a fixed measurement set-up and both incident laser beams have a direction perpendicular to the wafer surface or substrate surface, meaning incident at a zero angle relative to the wafer surface normal. An important difference between the excess carriers and the background carriers—which also create an index gradient—is that the excess carrier concentration can be modulated. A slow modulation of the pump laser, typically at 1 kHz, is used to allow the reflection of the probe laser signal by the excess carriers to be detected using phase-locked methods while maintaining quasi-static conditions. The modulation and the diffusion of the generated excess carrier in the semiconductor substrate 1 are in phase with the modulation of the pump laser.
The reflected probe power is given by the following theoretical formula, which is given by P. Borden, et al in “Carrier Illumination Characterization of Ultra-Shallow Implants”, in Handbook of Silicon Semiconductor Metrology, edited by A. C. Diebold, (Dekker Inc., New-York, 2001), 97, hereby incorporated by reference in its entirety:
                                          E            r            *                    ⁢                      E            r                          =                              r            s            2                    ⁢                      E            0            2                    ⁢                      {                                                                                1                    -                                                        ︸                    A                                                  ⁢                                                                            β                      n                                        ⁢                                          t                      2                                                                            r                    s                                                  ⁢                                  (                                                                                    N                        surf                                                                    ︸                        B                                                              +                                                                                            ∫                                                      0                            +                                                    ∞                                                ⁢                                                                              cos                            ⁡                                                          (                                                              2                                ⁢                                                                                                                                  ⁢                                                                  kn                                  Si                                                                ⁢                                z                                                            )                                                                                ⁢                                                                                    ⅆ                                                              N                                ⁡                                                                  (                                  z                                  )                                                                                                                                                    ⅆ                              z                                                                                ⁢                                                      ⅆ                            z                                                                                                                      ︸                        C                                                                              )                                            -                                                                                          β                      p                                        ⁢                                          t                      2                                                                            r                    s                                                  ⁢                                  (                                                                                    P                        surf                                                                    ︸                        D                                                              +                                                                                                                                                      ∫                                                              0                                +                                                            ∞                                                        ⁢                                                                                          cos                                ⁡                                                                  (                                                                      2                                    ⁢                                                                                                                                                  ⁢                                    k                                    ⁢                                                                                                                                                  ⁢                                                                          n                                      Si                                                                        ⁢                                    z                                                                    )                                                                                            ⁢                                                                                                ⅆ                                                                      P                                    ⁡                                                                          (                                      z                                      )                                                                                                                                                                        ⅆ                                  z                                                                                                                                              )                                                ⁢                                                  ⅆ                          z                                                                                            ︸                        E                                                                              }                                ⁢                                                                  ⁢                wherein                                                                        (        2        )                                                      β            n                    =                      -                                          q                2                                            2                ⁢                                                                  ⁢                                  m                  n                                ⁢                                  ω                  2                                ⁢                                  ɛ                  0                                ⁢                                  ɛ                                                                    ⁢                                  ⁢        and                            (                  2          ⁢          a                )                                          β          p                =                  -                                    q              2                                      2              ⁢                                                          ⁢                              m                p                            ⁢                              ω                2                            ⁢                              ɛ                0                            ⁢                              ɛ                                                                        (                  2          ⁢          b                )            and wherein                E0 and Er are respectively the incident and reflected probe signal electromagnetic field,        rs is the reflection coefficient at the air-substrate interface (in particular for a Silicon interface rs=−0.549),        βn and βp are negative electron- and hole-related constants which involve, among others factors, electron and hole effective masses,        t is the transmission coefficient at the air-substrate interface,        Nsurf and Psurf are the surface electron and hole excess carrier levels,        k=2π/λ is the field propagation constant in vacuum,        nSi is the substrate material index of refraction (in particular the silicon index of refraction nsi=3.435 at 980 nm),        z is the depth defined from the front surface into the semiconductor substrate,        N(z) and P(z) are respectively the electron and hole excess carrier profiles,        q is the elementary electron charge,        mn and mp are the electron and hole optical effective masses,        ω is the angular frequency (ω=k.c, where c is the speed of light),        ∈0 and ∈ are the dielectric constants of vacuum and the semiconductor substrate material, e.g. silicon, respectively.        
In equation (2), 0+ refers to the semiconductor side of the air-semiconductor interface, meaning that the integral is taken from immediately beneath the semiconductor surface into the bulk of the semiconductor substrate.
Equation (2) can be written as:power=constant(A−[B+C]−[D−E])  (3)
Whereby:                The A-component represents the reflection of the probe laser beam at the air-semiconductor interface. This is a constant term that is independent of the modulation of the pump laser.        The B-component represents the reflection of the probe laser beam near the surface by dopant related excess electrons. This component is modulated by the modulation of the pump laser. The integral ranges from 0 to 0+, indicating that the large value of the derivative dN(z)/dz at the air-semiconductor interface is being accounted for.        The C-component represents the reflection of the probe laser beam in the bulk, meaning the reflection by excess electrons in the region of the active dopant profile away from the surface. This component is modulated by the modulation of the pump laser. The integral ranges from 0+, which is just underneath the surface, into the bulk of the semiconductor material, indicating that the large value of the derivative dN(z)/dz at the air-semiconductor interface is not accounted for and only changes of this derivative of the excess electron profile in the bulk are taken into account.        The D-component represents the reflection of the probe laser beam near the surface by dopant related excess holes. This component is modulated by the modulation of the pump laser. The integral ranges from 0 to 0+, indicating that the large value of the derivative dP(z)/dz at the air-semiconductor interface is accounted for.        The E-component represents the reflection in the bulk, meaning the reflection by excess holes in the region of the active dopant profile away from the surface. This component is modulated by the modulation of the pump laser. The integral ranges from 0+ into the bulk of the semiconductor material indicating that the large value of the derivative dP(z)/dz at the air-semiconductor interface is not accounted for and only changes of this derivative of the excess hole profile in the bulk are taken into account.        
Since the first term (A) in equation (2), i.e. the surface reflection in the absence of any carriers, is a pure dc component, only the second (B+C) and third (D+E) modulation related terms in equation (2), which follow the pump laser modulation, represent the actual CI-signal.
The first part (B, D) of the second term (B+C) and third term (D+E) in equation (2), involving Nsurf and Psurf, are termed the (modulation-related) near-surface components. It has been found by the inventors that these near-surface components (B, D) can significantly contribute to the total signal if the peak concentration level of the dopant profile drops below 1020/cm3. This is due to the longer Auger lifetimes for lower doping levels. Consequently, a high CI-signal is measured on lowly doped bulk substrates. The presence of the near-surface component complicates the extraction of the dopant interface (unction) depth position from CI-signal versus depth response curves and/or signal versus pump laser power curves for unknown structures, because of the significant dependence of the position of these response/power curves on the near-surface component contribution.
The above-described problem is illustrated in FIG. 2. FIG. 2 illustrates correlation curves of the CI-signal, taken at a predetermined pump laser power, which in the example given may be 75 mA, versus SIMS junction depth at a predetermined concentration, in the example given 1e19/cm3, for CVD grown layers (box-profile) with different peak carrier levels. A graph such as in FIG. 2 can be created by performing the following subsequent steps:                creating a dopant profile in the semiconductor substrate 1 with a known peak/surface concentration for one type of species,        measuring the CI-signal of this dopant profile for a predetermined pump signal power, for example, a pump signal power of 75 mA,        measuring the junction depth at a selected concentration level using SIMS,        plotting both points (SIMS depth at selected concentration level, CI-signal at selected power) on FIG. 2,        repeating the above sequence measuring samples having a dopant profile for this species with the same peak concentration but various depths to create a correlation curve,        repeating the above sequence for different types of dopants or concentration levels to create additional correlation curves.        
In FIG. 2, curve 6 is valid for CVD grown layers with a dopant concentration of 1e19 cm−3, curve 7 for CVD grown layers with a dopant concentration of 5e19 cm−3, curve 8 for CVD grown layers with a dopant concentration of 1e20 cm−3 and curve 9 for CVD grown layers with a dopant concentration of 3e20 cm−3. It has to be noted that these CVD grown layers were only active for about 50% of their nominal peak dopant value, based on Four Point Probe measurements. This means that, for example, a dopant concentration of 5e19 cm−3 would result in an active dopant concentration of 2.5e19 cm−3. When measuring the CI-signal for an unknown doping profile in the CVD grown layers, it is impossible to extract the corresponding junction depth from FIG. 2, as long as one does not know the exact peak carrier concentration level, i.e. one does not know which correlation curve to take.